In recent years, non-volatile memories that are data-rewritable semiconductor devices have been widely used for various electronic devices such as portable telephone devices and digital cameras. As an example of a non-volatile memory, U.S. Pat. No. 6,011,725 discloses a SONOS (Silicon Oxide Nitride Oxide Silicon) flash memory that includes virtual ground memory cells that are symmetrically operated, with the sources and the drains being switched.
In this flash memory, bit lines also serve as sources and drains, and are formed with a diffusion layer embedded in a semiconductor substrate. Word lines also serve as control gates. Data writing in a memory cell is performed by applying a high voltage to the drain and the control gate of the transistor that forms the memory cell, and injecting hot electrons to a trapping layer that is a charge accumulating layer. As the writing is performed while the source and the drain are switched, two bits can be stored in each one memory cell. As electrons are injected and captured in the charge accumulating layer, the threshold voltage of the transistor that forms the memory cell becomes higher. Reading of data from a memory cell is performed by applying a predetermined voltage to the gate of the transistor, and detecting the current flowing between the source and the drain.
In a NAND flash memory, which is one type of flash memory, writing and reading are collectively performed for each one page (2 Kbytes, for example). Therefore, a NAND flash memory has a page buffer that holds page data and collectively writes page data from the page buffer into the memory cell array. Also, such a NAND flash memory collectively reads the page data from the memory cell array into the page buffer. A NAND flash memory normally includes memory cells each having a floating gate as a charge accumulating layer. Writing in a memory cell is performed by generating a high potential between the control gate on the floating gate and the substrate so as to cause a FN tunneling phenomenon. Accordingly, a large amount of data of one page can be written at once.
Japanese Patent Publication No. 11-273388 discloses a flash memory in which the bit lines to which voltage for data writing is to be applied are precharged before data is written in memory cells.
When data writing, data reading, or data erasing is performed in the virtual ground memory cells in a flash memory, the voltage to be applied to the memory cells should be kept constant, leakage current between the memory cells should be restrained, and the precharging of the bit lines connected between the memory cells should be performed quickly.